The use of spectrometry in analytical laboratories for measuring physical and analytical properties of materials is a well established art. Raman spectrometry is one such technique that can provide qualitative and quantitative information about composition and/or molecular structure of chemical substances. When incident radiation interacts with matter it may undergo a process called scattering. Scattered radiation may be elastic, in which the incident wavelength is unchanged in the scattered radiation, or inelastic, in which the scattered radiation has different wavelengths than the incident radiation. In one form of inelastic radiation scatter, referred to as Raman scattering, incident photons are scattered with either a gain or loss of energy. The energy difference between the scattered and incident radiation is commonly referred to as the Raman shift. The resultant Raman shift spectrum provides the energy of various molecular vibrational motions and conveys chemical and molecular information regarding the matter studied. The Raman scattering effect is extremely weak; typically a few Raman scattered photons exist among millions of elastically scattered photons.
Determining the constitution of a chemical composition or monitoring the progress of a chemical reaction is frequently carried out with materials situated in inhospitable environments. Analyses of process streams under conditions of high temperature and/or pressure or in the presence of corrosive substances or powerful solvents are often required. It may be necessary, for example, to follow the progress of a reaction forming polymers from lower molecular weight reactants in a high temperature continuous or batch process. Similarly, it may be desirable to monitor as a function of time the composition of batch reactions or volatile materials at a distillation head. Spectrophotometric apparatus such as a spectrograph and a radiation source can be situated in a location remote from materials such as polymer-forming compositions and distillation mixtures that are to be analyzed in situ, the sampling site being connected to the apparatus by radiation conduits comprising optical fibers.
Of course, the method of the present invention is not limited to use only in harsh environments characterized by, for example, higher temperatures. Quantitative in situ Raman spectrometric measurements in accordance with the invention may be carried out, assuming the availability of suitable optical probes, in diverse environments, including living organisms.
A polyester is a synthetic resin that contains ester linkages in the main polymer chain. Commercially valuable polyesters, useful for clothing fibers, container packaging, etc., are manufactured from various reactants. For example, they may be produced by esterification of dicarboxylic acids with diols, transesterification of dicarboxylic esters with diols, or self-condensation of hydroxycarboxylic acids.
Achieving particular end-use properties of a polyester requires vigorous control of the component ratios or composition of the materials in the reaction vessel during manufacture. Small changes in initial composition can dramatically affect the usefulness of the final polyester product. Control of the conversion of the ester or acid end groups, depending on the use of diesters or diacids, to the reactive hydroxy end groups is also required to ensure reliable finished polyester product characteristics. Low conversion during the first stage reaction limits the reactivity during the polycondensation reaction and adversely affects the ultimate end use properties of the polyester material. For this reason, it is extremely important to know the conversion or extent of the first stage reaction. Other critical composition control parameters include the amounts of each diacid and diol moiety, the ratio of total diols to total diacids and/or diesters in the reaction vessel, and the degree of polymerization, molecular weight, or size of the polymer chain.
During manufacturing, the chemical constitution of materials in the reaction vessels may be determined by removing a small sample for analytical testing in a remote laboratory. Commonly used analytical methods may be used to provide an indication of the extent of the first stage reaction, molar amounts of reactants and products in the mixture, and the extent of reaction. Laboratory methods commonly used to obtain compositional information of the extracted samples include nuclear magnetic resonance (NMR) spectrometry, gas chromatography (GC), and liquid chromatography (LC). These methods require the extracted sample to be dissolved and in some cases derivatized. NMR methods provide reliable information; however, the required instrumentation is expensive and complex and the sample must be properly prepared prior to measurement.